Hammer flight time aligning system for impact printers

ABSTRACT

A closed loop system is provided in an impact printer wherein a variable delay circuit is introduced into the command input to a hammer of the printer. The introduced delay time between the command input for the hammer to strike and the time at which the hammer actually is released or actuated to travel during its flight time from its normal position into impacting relation with the printer anvil, is adjustable so that the time of impact may be corrected to coincide with a desired timing relation, i.e. the hammer may be &#39;&#39;&#39;&#39;aligned.&#39;&#39;&#39;&#39; All hammers of the printer are similarly aligned and in order to accommodate for the fact that the actual flight time of any hammer will vary from strike-tostrike, provision is made for determining the average of several actual flight times and for correcting the delay time in accord with this average.

Unite States Palombo HAMMER FLIGHT TIME ALIGNING SYSTEM FOR IMPACTPRINTERS Primary Examiner-Edgar S. Burr Assistant Examiner-Edward M.Coven [75] Inventor. Gaston Albert Palombo, Dan outm, Attorney Agent orFirm Fred Jacob France [73] Assignee: Compagnie Honeywell Bull (Societe[57] ABSTRACT Anonyme), Paris, France I d A closed oop system is provied in an impact printer [22] 1973 wherein a variable delay circuit isintroduced into the [21] Appl. No.: 409,024 command input to a hammer ofthe printer. The introduced delay time between the command input for the30 hammer to strike and the time at which the hammer 1 Forelgn ApphcatmPnomy Dam actually is released or actuated to travel during its Oct. 26,1972 France 72.38060 time from its normal p iti n into impactingrelation with the printer anvii, is adjustable so that the Cl 101/9114,time of impact'may be corrected to coincide with a [51] Ill. Cl 134M7/08 dggirgd timing relation, i e, the hammer may be Fleld of Searchaligned." All hammers of the printer are similarly 340/1725 aligned andin order to accommodate for the fact that the actual flight time of anyhammer will vary from References Cited strike-to-strike, provision ismade for determining the UNITED STATES PATENTS average of several actualflight times and for correct- 3,312.174 4/1967 Cunningham 101/93 c ingthe delay time in accord with average- 3.602,i38 8/1971 Batomb 101/93 C3.662.389 5/1972 Janis 340/365 23 Clams 7 Drawmg F'gms Q D E L iDEP mock83 HF 7L 1 i +22 66 i w Bey |CAR1 T P Counter Shopen PATENIEBMARZSIBYS(SHEETIDFG cw U FiG-1 HAMMER FLIGHT TIME ALIGNING SYSTEM FOR IMPACTPRINTERS BACKGROUND OF THE INVENTION Modern impact printers include awriting medium in front of which passes a type carrier (i.e. a rotatingdrum, a linear support for the characters) which carry out a periodicalmovement so that each character to be printed is offered to eachprinting position of a print line. A strike organ is associated witheach printing position which releases an associated print hammer at thedesired moment, i.e. when the character to be printed is in thecorresponding printing position. Generally this release occurs due tothe action upon the hammer by an electromagnetic or electrodynamicactivator.

The hammers of the impact printers are usually freeflight ones, so as toavoid too long an impact time and early wear of the contact material Thereal flight time t of the hammer is that time from the instant in whichthe activator is ordered to release the hammer and the exact moment atwhich the hammer strikes the character support.

For the same hammer this time t,. changes from strike to strike. Forthis reason, a mean flight time is defined as z,.,,,. This time t,.,,,is different for different hammers and varies for the same hammer in thecourse of time. As the main reasons for these variations one may cite:

wear of the hammer faces entailing a change of the distance to becovered.

variation of the impact forces of the activator due to mechanicalfrictions, variation of the electrical voltage, of the temperature,etc,;

friction effects if the hammer is guided in flight;

- expansions;

sticking of the activator on the hammer, etc.

One must, however, realize that the mean flight time changes slowly sothat over a period of a few days, it essentially remains constant andthat this is characteristic of each hammer.

However, the means flight time will change significantly over longerperiods of time so that if it were not corrected, an ensuingdisalignment of the strike will result.

In current practice, an alignment operation is carried out duringmanufacture of the printer, and subsequently during maintenance of themachine.

In the alignment during manufacture a first alignment operation consistsof ascertaining by means of a bar provided with a shock absorber,functioning as the printer anvil, if the time intervals of the flight Iin relation to a mean value I, are not too great.

One records the flight times, i.e. with an instrument which has a Gausscurve for the flight times t as wellas the mentioned intervals. Thestrike motors whose hammers display intervals which are too large areeliminated and one proceeds with the second alignment operation.

The second alignment operation consists of manually changing thephysical parameters which affect the actual flight times of the selectedhammers, measuring the mean flight time by means of a bar functioning asthe printer anvil, and repeating these steps until a satisfactoryalignment is achieved. It is obvious that this procedure is iterativeand can only be performed ap proximately for it is very difficult todetermine accurately the average of a time with instruments usedhabitually for this type of measurement (i.e. an oscilloscope) and ashock absorber bar. To align a machine completely several hours arerequired which is obviously costly.

BRIEF SUMMARY OF THE INVENTION Since it is next to impossible to modifyor control the mechanical and physical causes of the variation of themean time of flight directly, the present invention involves theintroduction of a new value, namely, the mean fictitious flight timeTwhich may be adjusted for each hammer and may be made identical for thesystem. The mean fictitious flight is defined as the sum T t,.,,, rwhere 2 is the actual mean flight time and I is a delay time which isadjustable for each hammer.

This delay time is encompassed within that time period between theinstant at which the logic control circuitry of the printer commands thestrike and the instant at which the activator in fact receives theenergy necessary to release or actuate the hammer. The inventioninvolves a system in which the time of the fictitious flight T isidentical for all hammers by individual control of the time t for eachhammer.

The type support may be either a rotating cyllindrical drum or anendless type-carrying chain, mounted between two pulleys rotating onparallel axes, said hammer impacting against an anvil during each strikethrough the intervening selected type'carrying finger and the writingmedium.

In a particular mode of embodiment of the invention. the impactdetector, in the case of chain printers. consists of a series ofpiezoelectrical crystals positioned in the anvil. The impact signalproduced by a crystal is shaped by a suitable wave shaping circuitbefore being sent to the mean flight time measuring device.

The invention will be better understood in the course of the followingdescription, with reference to the attached drawings and offering forpurposes of explanation, without limitation, a mode of embodimentaccording to the invention.

BRIEF DESCRIPTION OF THE DRAWING FIGURES FIG. 1 represents in diagramform an example of the character support of a printer whose hammers arealigned by the device according to the invention;

FIG. 2 is a block diagram of the device according to the invention;

FIG. 3 is a timing diagram of signals describing the operation of thedevice of FIG. 2;

FIG. 4 exhibits more in detail the block diagram of FIG. 2;

FIG. 5 is a timing diagram of signals illustrating the operation of thedevice of FIG. 4-;

FIG. 6 is a timing diagram showing a possible division of thetimer'pulse as a pulse of fixed delay" and a pulse of variable delay;and

FIG. 7 illustrates a modification of the invention.

DETAILED DESCRIPTION OF THE INVENTION 7 The invention will be betterunderstood when the principles of construction and the operation of aprinter of a type-carrying chain are called to mind.

FIGS. 1a and lb show respectively a perspective and a plan view of animpact printer including such a typecarrying chain 100. The chain isillustrated mounted in the printing machine on two pulleys 101A and 1013whose axes Yl Y'l and Y2 and Y'2 are verti- I cal. To facilitate theunderstanding of FIG. 1a, only a part of the chain 100 is shown with itstype-carrying tongues 111 to 120, (ten in the case of thisillustration). The latter are intentionally oversized in relation to thechain 100 so as to make FIG. la more distinct. In the same manner, acertain number of hammers 120 to 130 are shown in a very schematic formin FIG. 1b together with associated actuators 120 to 130'. This endlessbelt 100 passes in a linear manner with constant speed between thehammers 120 to 130, on the one hand, and the writing medium 102 and afixed anvil 103 on the other hand. I

The speed of the chain 100 is adjustable and depends on the requiredprinting speed for the printing machine. Synchronization apertures 131to 140 are associated with the respective fingers 111 to 120, in suchfashion that each finger corresponds with a single synchronizationaperture.

As shown in FIG. la, the synchronization holes are located between thetype-carrying fingers. Thus the aperture 131 is positioned between thefingers 110 and 111, the hole 132 between the fingers 111 and 112 and soon. If the chain 100 passes in the direction of the arroes F1 and F2, asillustrated, the aperture 131 is associated with the finger 111, thehole 132 with the finger 112 and so on.

The type-carrying chain 100 further provides an initial chain hole 104,positioned on a different alignment level than thatof thesynchronization holes.

A synchronization pickup C1 is disposed at the level of thesynchronization holes and furnishes a pulse whenever a synchronizationhole 131-140 passes before it. This pulse is sent to the conventionalstrikeorder logic circuits of the printer.

Associated with and at the level of the initial opening 104 of the chainis a pickup C2 that emits a pulse when this aperture 104 passes beforeit, which pulse is also sent to the logic circuits. I,

The combination of the two signals emitted by the pickups Cl and C2makes it possible in a known manner to identify by said logic circuitsall the synchronization openings and, consequently, all the fingers.When one keeps in mind the relative geometrical constellation formed ofthe pickups, the hammers and the fingers, one knows always the relativepositions of the hammers with respect to this or that type-carryingfinger so that one may order the desired strike accordingly, all as isconventional and well known in the art.

The device according to the invention is represented in the form of ablock diagram in FIG. 2.

vIn FIG. 2 there may be seen the different essential composing elementsof the device according to the invention, i.e.:

l. The timer generator 1 which is a digital monostable;

. The strike amplifier 2;

. The hammer 3;

. The impact detector 4;

. The means flight time measuring device 5; The subtracting circuit 6;

. The order corrector circuit 7; and

. The time base 8.

To better appreciate the principle of operation of the present device offlight time alignment, it is necessary to call to mind the the operationof a hammer in the course of a strike, with the understanding that thisfunction is the same for all the others.

When one of the synchronization openings 131 to passes in front of thesynchronization pickup C1, the strike-order logic circuits send thepulse DEP to the delay monostable l which corresponds with the hammerchosen by the said circuits. This pulse DEP, represented in FIG. 2,rises to logic 1 at the instant T., and drops again to zero logic at themoment T 'at which time the delay monostable 1 outputs. The lattersupplies the pulse DEL which rises to logic 1 at the instant T and fallsagain down to logic zero in the instant t, at which the strike amplifier2 furnishes the order pulse MART to the hammer 3 which order pulse dropsagain to zero in the instant t The hammer 3 is actuated at the time tand strikes the character at the instant When recapitulating thenotations initially used one may write that the actual flight time ofthe hammer is t, -1 while the fictitious flight time of the hammer IS isthe adjustable duration of the pulse supplied by the monostable 1, alsocalled delay time.

Since the flight time I for the same hammer changes from strike tostrike the mean flight time I,.,,, is introduced.

This value t,.,,, not only changes from hammer to hammer but also variesin time for the same hammer, as was explained above.

The mean fictitious flight time is, therefore, introduced The automaticalignment operation performed by the device according to the inventionthus consists in rendering identical the quantity T for all hammers byad justment of the variable quantity (r I t,,.

The theoretical alignment operation schedule consists in the followingfor each hammer:

I. perform the same number n of strikes in each schedule; and

2. measure in each strike the corresponding t,,.

If i is the order of the order of the strike (1' n) and 1,, (i) is thecorresponding fictitious flight time, the mean fictitious flight timewill be after n strikes .3. in calculating T; 4. in comparing it with areference flight time T and in computing the difference;

T TR e;

5. in adding or subtracting T.. from 1,

In practice, the device of FIG. 1 accomplishes the alignment operationof the hammer 3 in the following way.

The time base 8 delivers from the moment t on to the digital monostable1 a series of it set pulses H whose duration is roughly longer than themaximum time T assumed, in such a manner that between the instant inwhich the hammer strikes the anvil and the instant in which thefollowing pulse l-I begins, there is sufficient time to permitstabilization of the hammer. One examines what occurs during each pulseH The monostable 1 supplies the pulse DEL of a duration r,, to thestrike amplifier 2 which delivers the pulse MART, permitting by means ofan actuator of a known type (i.e., electromechanical) to release oractuate the tan.

Each flight time 1,;(1') and consequently T is expressed in binary formin time units h of the signal UTI furnished by the time base 8 to thedevice 5. The value of T is sent to the subtractor circuit 6 where thereference flight time T is subtracted from T. T is also expressed inbinary form as units of h (see FIG. 3) with t T where 1 is defined asthe instant of the strike of a perfect hammer of the flight time t equalto T Thereafter, when one speaks about the flight times r,.,(i); T, T ofthe delay time I it will have to be understood that they are expressedin time units h. The difference T-T (expressed in binary form) is sentto the ordercorrector circuit 7 which transforms it into a signal E (seeFIG. 2) which is a sequency of very short pulses whose period is equalto that of signal UT2, supplied by the time base 8 (usually differentand below h) and whose number is equal to the duration of the errorwhich one has to add or deduct to (from) the delay time 1,, according tothe flight time T of the hammer being too short or too long in relationto T (T is represented too long in FIG. 3) The corrector circuit 7modifies by the sending of signal E to the triggered monostable 1 theadjustment of the latter in such a fashion that the mean flight time Tbe henceforth equal to T To better facilitate the understanding of thiscorrection operation the signals DEL. H1, MART, IMP, TVR are representedon the right side of FIG. 3 from the moment 1'0. Once the correction isachieved and the prime" index has been attached to the variable t toshow that the series of n pulses H1, hence of n strikes, is terminatedand that the correction of the delay time r has been accomplished by thedevice according to the invention, the values of the variable r equal to1' 1' the value of the time interval I' correspond tothe values 1,, rOne will find that the delay time and that the impact instant t'coincides with the instant 1' According to the above, it may be seenthat the time base 8 delivers the signal U TI. supplying the referencetime h to the measuring device 5, the sequence of pulses H, to thedigital monostable I and the signal & T2 to the corrector circuit 7.This time base 8 has thus an essential function in the operation of thedevice according to the invention. The different elements which make itup may be seen in FIG. 4. They are a first clodk 81, a bistable 82, anAND gate 83, a strike counter 84, a second clock 85 and an AND gatecircuit 86.

The first clock sends out from the moment t in which the pulse DEP dropsdown to zero an endless sequence of pulses H, (see FIGS. 3 and which issent to the first input of the circuit ET 83.

The bistable circuit 82 furnishes a pulse BIS which rises to logic 1 atthe instant t and falls down again to logic zero in the instant 1 inwhich the nth pulse of the sequence III drops to zero logic. Thus pulseBIS is sent to the second input of the AND circuit 83. One receives inthe output of the AND circuit 83 the sequence of pulses I'll, sent tothe first input of the AND circuit 86, to an input II of the digitalmonostable l and finally to the input of the strike counter 84. Thelatter registers a strike whenever one of the n pulses of the sequenceI-lll arrives there. The capacity of counting of the counter 84 is equalto n 2", i.e. at the nth pulse the counter sends a pulse CAR I to thesecond input of the bistable 82. This pulse CAR 1, when it drops againto zero logic in the instant t causes the pulse BIS to fall again tozero, while the pulse BTS rises to logic I. This pulse is sent to theinput H of the order corrector circuit 7.

The clock supplies a signal U TI giving the time unit h of reference tothe measuring device of the flight time 5, and to the second input ofthe ANd circuit 86. The signal U T Hll is received in the output of theAND circuit 86 (see FIG. 5) which is sent to the input 12 of the digitalmonostable ll.

In view of the fact that the functioning of the triggered monostable 3,of the impact detector 4, and of the measuring device of the meansflight time 5 is anal ogous, one considers from strike to strike of onepulse Hi to another one pulse H, i.e., the first. Furthermore, tofacilitate the understanding of FIG. 5 only two series of signals arerepresented, the one corresponding with the first pulse H the othercorresponding with the nth pulse H the instants t t,, l 1 being relativeto the first pulse, the corresponding times t 1, I 1 1 being relative tothe nth pulse.

As was mentioned previously, the monostable I delivers the pulse DEL tothe strike amplifier 2 which supplies by the pulse MART the forcenecessary for the release or actuation of the hammer 3. The descriptionand a detailed operation of the monostable I will be given later on, theunderstanding of the functioning of the latter is much easier if theoperation of the detector 4 of the mean flight time measuring device 5,the subtractor 6 and of the circuit 7 will have been explained.

The impact detector 4 includes the transducer 11 and the wave shapingcircuit 42.

The transducer 41 is positioned inside the anvil 103 of the printer andis of the type which permits accurate determination of the instant ofimpact. The transducer may take any conventional form such as apiezoelectrical crystal, microphone, strain guage, etc. The transducerll delivers a signal 'CIIOC which is shaped by the circuit 82 whichemits in its output the pulse IMP (FIGS. 3 and 6).

The mean flight time measuring device 5 includes the flip-flop circuit51 and the AND gate 52 with two inputs, the counter accumulator 53 of acapacity 2""*". the division circuit which performs n 2'".

The flip-flop circuit 81 receives at its first input the signal IMF andat its second input the pulse Hi to produce the output signal TVM. apulse which rises to logic l in the instant t (i.e. at the same momentas the rise of the pulse III) and drops again to zero logic in theinstant 1 in which the pulse IMP rises to logic. I. The duration of thatpulse TVM is thus equal to 1 t um)- The signal TVM is sent to the firstinput of the AND gate 52, while the second input of this gate receivesthe signal U T1. In the output of the gate 52 a sequence TVM of qi U Tlpulses is thus received and this sequence is sent to the counteraccumulator 53 which records the number qi.

What was described with respect to the first strike (and the signalswhich concern it) applies to the (n-l) other strikes and one receivesfor example:

At the end of n trikes, the counter 53 will have totaled the number(ql-q,-+ q,,) (t,.,(l) t5,

Obviously, the counting capacity of the counter 53 is such that thepreceding number does not completely fill it. The capacity of thecounter 53 is equal to 2 2"X2" in which the number 2 is chosen so that 2h be higher than the maximum fictitious flight time t assumed for thehammers of the printer.

Thus, the counter 53 supplies the dividing circuit 54 in its (m-l-p)parallel outputs the number (q q, q,,). The circuit 54 performs thedivision of this number by 2'" n and delivers, thereafter, a numbert,.(i)/(n lz) equal to the mean flight time T of the hammer underconsideration. This number is sent from p outputs of the circuit 54 tothe subtractor 5 in p bits; the latter receives in binary form thenumber equal to the reference flight time T delivers the number Te toits p outputs if T, (T, T is positive or the complement of T if T,, isnegative, i.e., 2" T The order-corrector circuit 7 includes the AND gate72 having two inputs, the error abstractor 75 of a capacity of 2" andthe monostable of delay transfer 76.

The monostable 76 receives the signal m of the bistable of the bistable82 and delivers a signal TRANSF which rises to logic 1 in the i instanti (at the same time as m5) and falls back to zero in the instant tdefined arbitrarily. This signal TRANSF is sent to one of the AND gate72 of which the p other inputs receive the number Te (or its complement2 Te if Te It may be seen then that this gate 72 is validated by thesignal TRANSF from the moment t i.e. when the hammer has finished'itsseries of'n strikes. This difference Te is sent by the gate 72 inparallel to all p inputs of the up-down counter 75. The signal TRANSF isalso sent to the first input of the corrector flip-flop 73, while theother input receives the signal BOR which is produced when the counter75 is emptied. The flip-flop 73 delivers the correction signal COr whichincreases to logic 1 in the instant t and falls again to zero logic assoon as the pulse BOR is received. This signal COR is sent at the sametime as the signal U T2 (whose period is generally below h so that thecorrection operations may be carried out very rapidly) to the AND gate74 at whose output the signal Eis received, a sequence of very shortpulses. This signal, sent to the down counteinput of the counter 75causes the latter to move back a digit at each E pulse received untilthe counter 75 is emptied. Since the latter, before receiving the signalE, contained the value Te of the difference of the flight times (or itscomplement to 2", as could be seen above), one realized that the signalCOR is of such duration that the number of E pulses is equal to Te if Teis positive or its complement 2' Te is negative.

The digital monostable 1 includes the delay counter of the gate 17 tothe storage counter 14. This presettingnumber is arbitrarily chosenbetween 0 and 2". The counter 14 presets in turn the counter 13 at thevalue PP by means of the AND circuit 16 because at this time the signalDEE is high.

b. lntermediate phase (during each pulse H The counter 13 receives thesignal UT H, at its conductor 12 stemming from the AND circuit 86 andconsisting of a predetermined number of pulses from the clock 85. Ateach pulse received it moves forward one unit until its capacity 2" isfilled by the preset number PP plus the predetermined number of clockpulses (instant t,). In this instant I, the counter sends out the pulseCAR which is sent to an input of the delay bistable 15 which receives inthe other input the pulse H The bistable 15 changes its state and thepulse DEE goes up to logic 1, enabling the AND circuit 16 to reinsertthe presetting number PP into the counter 13. It thus may be seen thatsince the capacity of the counter 13 is 2" the number 2" PP is equal tothe time of the delay 1 0)- c. Final phase (after the n pulses H1). Atthis time the signal EB goes low and the signal BIS goes high (FIG. 5)so that the gate 83 no longer is enabled and the monostable 76 isactuated to produce the pulse output signal TRANSF which is of aduration sufficiently long as to enable the ensuing transfer of theaccumulated or averaged error 'signal Te to be transferred to thecounter 75. This transfer is effected by the gate 72. At the same time,the signal TRANSF sets the bistable 73 such that its state changes toproduce the output signal COR, thereby allowing the pulses from theclock 85 to down-count the counter 75. The number of clock pulses (clock85) required to empty the counter 75 is also applied to the countinginput of the counter 14 to modify its store so that the counter 14 willnow store that number which either is PP Te or PP Te dependent uponwhether the number Te 0 or whether Te 0, in which latter case thecomplement of Te is present in the counter 75 at the end of the nthstrike. The delay T is therefore either increased or decreased to thenew value I' (FIG. 3) so that the corresponding hammer will behenceforth actuated at that new time ['2 which in the average assuresthat the moment of hammer impact occurs at t'4. As noted. the counter 75when emptied produces the output signal BOR which changes the state ofthe binary 73 to terminate the gateenabling signal COR.

Two cases must be examined:

1. If Te 0, the counter 14 counts then the number (PP Te): The delaybecomes then 2" PP Te and T is thus reduced.

2. If Te 0 the counter stores PP 2" Te or PP Te and the delay-t becomes2 PP Te. It has, therefore, increased.

In the two cases, the new value of delay is immediately transferred tothe delay counter 13 (by validation by DEE of the gate 16).

In a preferred version of realization of the invention, the signal DELis divided in two signal DELV and DELF, as is shown in FIG. 6, ofrespective duration (t t and t and one receives always 2. After the nstrikes: the counter 13 receives the signal E at its input conductor 132thus to change the count stored therein. The operation-of the counter 13is then exactly identical with that of the storage counter 5 14 of thefirst version (see above).

After the nth pulse H1, the previously stored count PP in the counter 13is altered to a new value dependent upon the error term E, as describedabove in conjunction with FIG. 4. Since the period of the signal DELF isfixed and since the total time represented by according to the inventionwhich is shown on the right side of FIG. 6. As in FIG. 3, the primeindex attached to a variable E, means that a series of n pulses H henceof it strikes, is terminated, and that the duration n, of the delay timehad been modified from its value Te by the device according to theinvention, to render the values of 1 equal to t,, t' t correspondingwith the preceding value I t t If one has (t, 1' t,, =t,,-Te=(t t)-Te=(t' t' )+(t -t' with To accomplish the divison of the signal DEL inDELV and DELF and to carry out the correction of the signal DELV, thetime base 8 and the digital monostable 1 are modified as indicated inFIG. 7; the other elements of the correction device according to theinvention remain unchanged in relation to the first described version.

The time base 8 includes always the elements 81, 82, 83, 84, 85. Oneadds to it the pulse generator 87 which receives the signal U T1supplied by the clock 85 and furnishes a sequence UT H2 of n 2* pulsesduring the time duration of the signal U T1, the time duration of thesignal H1. The triggered monostable 1 comprises besides the delaycounter 13 of the counting capacity the sequential periods of DELV andDELF determine the delay time r it is possible that the error term Ewill be so great as to indicate that the initial count in the counter 13should be equal to or greater than 2". This, of course, represents anambiguity and indicates simply that the period of the signal DELF is toolittle. in other words, the monostable 18 must be adjusted to increasethe period of the signal DELF, which may be effected 20 by manualadjustment.

in order to provide means by which a correction due to the error term Ewould cause the capacity 2 of the counter 13 to be exceeded, the masterclock 9 is employed as shown in F IG. 7. The master clock 9 provides thebasis timing format for the system from which all CG whose output at ALprovides a suitable alarm such 2p, the delay bistable 15 and theanalogue monostable 18. Obviously compared to the first version of theinvention the storage counter 14 is omitted.

The operation of the digital monostable is now the following: W

1. During each of n strikes: The counter 13 is preset to start its countat some number PP. The value of the variable delay (r t is, therefore,2" PP. The counter 13 receives at the input conductor 131 the pulsesequence UT H2. It thus advances by one unit at each pulse received. Itwill be recalled that the pulse generator 87 produces 2 pulses duringthe period of the pulses H1 so that at some time t the counter is filledand at the instant r produces the pulse CAR 2 to the first input of thedelay bistable 15 whose second input receives the signal H The delaybistablev 15 transmits the pulse DELV to the monostable 18 whichdelivers in response to the latter the pulse DELF of the fixed duration(l -I In the meantime the signal H1 is still up so that the counter 13continues to receive the pulses UT H2 after being filled and emptied andtheirby counts toward the number PP. Since the capacity of the counteris equal to the number of pulses of the signal UT H2 and since theperiod of the signal UT 1 is approximately equal to that of H1, thecounter 13, at the end of each of the n pulses l-l,, is again preset tothe count PP, without being filled again by the time the signal H1 goeslow.

as a bell, a light, etc. which informs the operator that manualadjustment of the monostable 18 is required to increase the period ofthe signal D'ELF.

The elements 1, 5, 6, 7 and 8 of either system as described above, inother words the logical system of computation and correction of thequantity T, are preferably common to all hammers, the alignmentoperations carried out during the periods of upkeep and maintenance ofthe printing machine being affected by the one system and the hammersare aligned one after another. However, it is evident that an alignmentsystem could be aligned simultaneously. It is also possible to performthe alignment of the hammers while the printer carriers on its normalwork.

What is claimed is:

1. in an impact printer having a printing anvil, a plurality of hammersnormally spaced from said anvil, type carrier means disposed betweensaid hammers and said anvil and including means for causing relativemovehammer impact against the anvil is desired, actuator means forcausing each said hammer selectively to travel into impacting relationto said anvil, and timing means for initiating actuation of each saidactuator means in response to attainment of a predetermined relativeposition of such hammer and the type carrier means which is in advanceof attainment of a corresponding particular alignment the improvementwherein;

said timing means includes variable delay means for selectively varyingthe time of initiating movement of each said hammer with respect to thetime of attaining the corresponding relative position of such hammer andthe anvil;

ing the time of impact of a selected hammer with said anvil; and

correction means connecting each detector means with an associatedvariable delay means for controlling said time of initiating movement ofthe selected hammer to cause such hammer to impact the anvil withessentially a predetermined time delay with respect to the attainment ofsaid predetermined relative position between such hammer and the anvilwhereby all hammers may be actuated to impact at the desired times.

2. ln an impact printer as defined in claim 1 wherein said correctionmeans includes means for averaging the flight time of the selectedhammer over a number of actuations thereof and, means for controllingsaid variable delay means according to such time average.

3. An impactjprinter as defined in claim 2 wherein said detector meansincludes an impact detecting transducer for each hammer, each suchtransducer being mounted in said anvil.

4. An impact printer as defined in claim 3 wherein said means foraveraging includes a count accumulator, a count pulse generator, and agate, said count pulse generator being connected to said countaccumulator through said gate and said gate being controlled by saiddetector means to pass pulses from said pulse generator in dependenceupon the actual time required for the selected hammer to impact theanvil.

5. An impact printer as defined in claim 4 wherein said means foraveraging also includes means for dividing the total number ofaccumulated pulses by said number of actuations of the selected hammer.

6. An impact printer as defined in claim 1 wherein said means foraveraging includes a count accumulator, a count pulse generator, and agate, said count pulse generator being connectedto said countaccumulator through said gate being controlled by said detector means topass pulses from said pulse generator in dependence upon the actual timerequired for the selected hammer to impact the anvil.

7. An impact printer as defined in claim 2 wherein said means foraveraging includes a count accumulator,

.a count pulse generator, and a gate, said count pulse generator beingconnected to said count accumulator through said gate and said gatebeing controlled by said detector means to pass pulses from said pulsegenerator in dependence upon the actual time required for the selectedhammer to impact the anvil.

8. An impact printer as defined in claim 7 wherein said means foraveraging also includes meansfor dividing the total number ofaccumulated pulses by said number of actuations of the selected hammer.

9. In an impact printer having a set of hammers, an anvil spaced fromsaid hammers and actuator means for the hammers whereby inherently toprovide an arrangement in which different hammers may required differenttime periods in which to reach impact with the anvil with respect to acommand signal input time to said actuator means, the combination of:

clock input means for providing a series of clock pulses in response tosaid command signal input;

delay counter means connected to said actuator means and having saidseries of clock pulses as an input thereto for delaying energization ofthe actuator means until a particular number of clock pulses occur; and

means for selectively altering said particular number of clock pulseswhich must occur before energization of said actuator means is effected.7

10. In an impact printer as defined in claim 9 wherein said clock inputmeans includes a free running clock and gate means enabled by saidcommand signal to pass said series of clock pulses to said delay countermeans.

11. ln an impact printer as defined in claim 10 wherein said delaycounter means has a selected count capacity and energizes said actuatormeans upon attainment of said count capacity, and wherein said means forselectively altering comprises means for inserting a variable count intosaid delay counter means.

12. In an impact printer as defined in claim 11 including detector meanscarried by said anvil for detecting the moment of impact of a hammerwith the anvil, gate means connected to said clock input means and tosaid detector means for averaging the number of clock pulses producedbetween the instant of actuator means energization and detector meansactuation over a selected number of actuations of a particular hammer,and wherein said means for inserting includes sub tractor meansconnected to said means for averaging for producing an error signalwhichis the difference between the average number of clock pulses and areference number of clock pulses.

13. In an impact printer as defined in claim 12 wherein said delaycounter means is updated by said subtractor means after every actuationof said particular hammer during said selected number of actuationsthereof.

14. In an impact printer as defined in claim 12 wherein said means forinserting includes a storage counter connected to said delay countermeans and gate means connecting said subtractor means to said storagecounter for updating the storage counter at the end of said selectednumber of actuations of a particular hammer.

15. A closed loop system for aligning the flight time of the hammer ofan impact printer, said closed loop system including said hammer,detector means for determining the moment of impact of said hammer withan associated anvil, logic circuit imput means for commandingenergization of said hammer at a successive number n of particulartimes, delay means connected to said logic circuit input means fordelaying actuation of said hammer with respect to each said particulartime by a delay time period r averaging means connected to said detectormeans for determining the average actual time period between actuationof said hammer and impact thereof over said successive number n ofhammer actuations, subtractor means connected to said averaging meansfor determining the difference between said average actual time periodand a predetermined reference time period, and corrector meansconnecting said subtractor means to said delay means for changing saiddelay time period t such that the period of time between the instant ofcommand signal input and the instant of hammer impact approaches aselected correct value.

16. A closed loop system as defined in claim 15 wherein said delay meansincludes a counter, said logic circuit input means includes clock meansconnected to said counter in response to command input, and wherein saidcounter is of fixed capacity and includes an output terminal foractuating said hammer upon reaching a count corresponding to said fixedcapacity.

17. A closed loop system as defined in claim 16 wherein said correctormeans is connected to said counter to insert a count therein.

18. A closed loop system as defined in claim wherein said delay meansincludes a counter and said corrector means is connected to said counterto preset same to a predetermined count which controls the delay time t19. A closed loop system as defined in claim 18 wherein said delay meansalso includes a monostable connected to said counter and actuatedthereby when said counter reaches its count capacity.

20. A closed loop system as defined in claim 19 wherein said logiccircuit input means includes clock means connected to said counter inresponse to command input.

21. A closed loop system as defined in claim 18 wherein said delay meansalso includes a bistable device connected to said counter and actuatedthereby when said counter reaches its count capacity.

22. A closed loop system as defined in claim 21 wherein said delay meansalso includes a bistable device connected to said counter and actuatedthereby when said counter reaches its count capacity.

23. A closed loop system as defined in claim 15 wherein said delay meansincludes a counter of selected count capacity and said corrector meansis connected to said counter to preset same to a predetermined countafter the nth hammer actuation, said logic circuit input means includingclock means connected to said counter in response to command input forcausing said counter to count, said delay time 1,, being controlled independence upon the time required for said counter to reach saidselected capacity from said predetermined count.

1. In an impact printer having a printing anvil, a plurality of hammersnormally spaced from said anvil, type carrier means disposed betweensaid hammers and said anvil and including means for causing relativemovement of said type carrier means between said hammers and said anvilperiodically to effect particular alignments of the type carrier meansrelative to the hammers and the anvil at the times of which alignmentshammer impact against the anvil is desired, actuator means for causingeach said hammer selectively to travel into impacting relation to saidanvil, and timing means for initiating actuation of said actuator meansin response to attainment of a predetermined relative position of suchhammer and the type carrier means which is in advance of attainment of acorresponding particular alignment the improvement wherein: said timingmeans includes variable delay means for selectively varying the time ofinitiating movement of each said hammer with respect to the time ofattaining the corresponding relative position of such hammer and theanvil; detector means associated with said anvil for detecting the timeof impact of a selected hammer with said anvil; and correction meansconnecting each detector means with an associated variable delay meansfor controlling said time of initiating movement of the selected hammerto cause such hammer to impact the anvil with essentially apredetermined time delay with respect to the attainment of saidpredetermined relative position between such hammer and the anvilwhereby all hammers may be actuated to impact at the desired times. 2.In an impact printer as defined in claim 1 wherein said correction meansincludes means for averaging the flight time of the selected hammer overa number of actuations thereof and means for controlling said variabledelay means according to such time average.
 3. An impact printer asdefined in claim 2 wherein said detector means includes an impactdetecting transducer for each hammer, each such transducer being mountedin said anvil.
 4. An impact printer as defined in claim 3 wherein saidmeans for averaging includes a count accumulator, a count pulsegenerator, and a gate, said count pulse generator being connected tosaid count accumulator through said gate and said gate being controlledby said detector means to pass pulses from said pulse generator independence upon the actual time required for the selected hammer toimpact the anvil.
 5. An impact printer as defined in claim 4 whereinsaid means for averaging also includes means for dividing the totalnumber of accumulated pulses by said number of actuations of theselected hammer.
 6. An impact printer as defined in claim 1 wherein saidmeans for averaging includes a count accumulator, a count pulsegenerator, and a gate, said count pulse generator being connected tosaid count accumulator through said gate being controlled by saiddetector means to pass pulses from said pulse generator in dependenceupon the actual time required for the selected hammer to impact theanvil.
 7. An impact printeR as defined in claim 2 wherein said means foraveraging includes a count accumulator, a count pulse generator, and agate, said count pulse generator being connected to said countaccumulator through said gate and said gate being controlled by saiddetector means to pass pulses from said pulse generator in dependenceupon the actual time required for the selected hammer to impact theanvil.
 8. An impact printer as defined in claim 7 wherein said means foraveraging also includes means for dividing the total number ofaccumulated pulses by said number of actuations of the selected hammer.9. In an impact printer having a set of hammers, an anvil spaced fromsaid hammers and actuator means for the hammers whereby inherently toprovide an arrangement in which different hammers may required differenttime periods in which to reach impact with the anvil with respect to acommand signal input time to said actuator means, the combination of:clock input means for providing a series of clock pulses in response tosaid command signal input; delay counter means connected to saidactuator means and having said series of clock pulses as an inputthereto for delaying energization of the actuator means until aparticular number of clock pulses occur; and means for selectivelyaltering said particular number of clock pulses which must occur beforeenergization of said actuator means is effected.
 10. In an impactprinter as defined in claim 9 wherein said clock input means includes afree running clock and gate means enabled by said command signal to passsaid series of clock pulses to said delay counter means.
 11. In animpact printer as defined in claim 10 wherein said delay counter meanshas a selected count capacity and energizes said actuator means uponattainment of said count capacity, and wherein said means forselectively altering comprises means for inserting a variable count intosaid delay counter means.
 12. In an impact printer as defined in claim11 including detector means carried by said anvil for detecting themoment of impact of a hammer with the anvil, gate means connected tosaid clock input means and to said detector means for averaging thenumber of clock pulses produced between the instant of actuator meansenergization and detector means actuation over a selected number ofactuations of a particular hammer, and wherein said means for insertingincludes subtractor means connected to said means for averaging forproducing an error signal which is the difference between the averagenumber of clock pulses and a reference number of clock pulses.
 13. In animpact printer as defined in claim 12 wherein said delay counter meansis updated by said subtractor means after every actuation of saidparticular hammer during said selected number of actuations thereof. 14.In an impact printer as defined in claim 12 wherein said means forinserting includes a storage counter connected to said delay countermeans and gate means connecting said subtractor means to said storagecounter for updating the storage counter at the end of said selectednumber of actuations of a particular hammer.
 15. A closed loop systemfor aligning the flight time of the hammer of an impact printer, saidclosed loop system including said hammer, detector means for determiningthe moment of impact of said hammer with an associated anvil, logiccircuit imput means for commanding energization of said hammer at asuccessive number n of particular times, delay means connected to saidlogic circuit input means for delaying actuation of said hammer withrespect to each said particular time by a delay time period td,averaging means connected to said detector means for determining theaverage actual time period between actuation of said hammer and impactthereof over said successive number n of hammer actuations, subtractormeans connected to said averaging means for determining the differencebetween said average actual time period and a predetermined referencetime period, and corrector means connecting said subtractor means tosaid delay means for changing said delay time period td such that theperiod of time between the instant of command signal input and theinstant of hammer impact approaches a selected correct value.
 16. Aclosed loop system as defined in claim 15 wherein said delay meansincludes a counter, said logic circuit input means includes clock meansconnected to said counter in response to command input, and wherein saidcounter is of fixed capacity and includes an output terminal foractuating said hammer upon reaching a count corresponding to said fixedcapacity.
 17. A closed loop system as defined in claim 16 wherein saidcorrector means is connected to said counter to insert a count therein.18. A closed loop system as defined in claim 15 wherein said delay meansincludes a counter and said corrector means is connected to said counterto preset same to a predetermined count which controls the delay timetd.
 19. A closed loop system as defined in claim 18 wherein said delaymeans also includes a monostable connected to said counter and actuatedthereby when said counter reaches its count capacity.
 20. A closed loopsystem as defined in claim 19 wherein said logic circuit input meansincludes clock means connected to said counter in response to commandinput.
 21. A closed loop system as defined in claim 18 wherein saiddelay means also includes a bistable device connected to said counterand actuated thereby when said counter reaches its count capacity.
 22. Aclosed loop system as defined in claim 21 wherein said delay means alsoincludes a bistable device connected to said counter and actuatedthereby when said counter reaches its count capacity.
 23. A closed loopsystem as defined in claim 15 wherein said delay means includes acounter of selected count capacity and said corrector means is connectedto said counter to preset same to a predetermined count after the nthhammer actuation, said logic circuit input means including clock meansconnected to said counter in response to command input for causing saidcounter to count, said delay time td being controlled in dependence uponthe time required for said counter to reach said selected capacity fromsaid predetermined count.